In recent years, display devices, typified by flat display devices, have advanced, and the resolution and the number of gray-scale levels have been increased so that more realistic video images having higher image-quality can be displayed. Further, the digitization of video signals transmitted to display devices have also made progress, and 6-bit to 8-bit data, typically, is allocated to each of color components composed of R (red), G (green), and B (blue) for each pixel.
If the number of bits of a video signal is equal to the number of bits that can be displayed on a display device, the input signal is basically used to display an image without carrying out any additional processing in the display device. However, the number of bits of a video signal is often different from the number of bits that can be displayed on a display device.
If the number of bits of a video signal is larger than the number of bits that can be displayed on a display device, the video signal is dealt with by using a method in which a lower-order bit(s) of the video signal are discarded, a dither method, or an FRC (frame rate control) method. On the other hand, if the number of bits of an image signal is smaller than the number of bits that can be displayed on a display device, the video signal is dealt with by using a method different from the above method in order to add an additional lower-order bit(s) (gray-scale extension).
This gray-scale extension is also used in the case where digital image processing according to the characteristics of a display device is performed in the display device. This gray-scale extension is performed, even when the number of bits of the video signal is equal to the number of bits that can be displayed on the display device, in order to improve the arithmetic precision, and after that the video signal is converted into the number of bits that can be displayed on the display device by using the above-mentioned dither method or the FRC.
Such gray-scale extension has not been limited to the use for a method for converting the number of bits of digital signals and for a method for improving arithmetic precision. If the number of bits of a digital signal is small, false contours appearing as contour lines (a phenomenon in which portions that are supposed to vary smoothly in terms of spatial gray-scale variation under normal circumstances are not perceived as varying smoothly but recognized as contour lines) becomes problematic in an area where the gray-scale varies smoothly like a gradation. Accordingly, the gray-scale extension is also used as a technique for preventing such a problem.
The gray-scale extension method is classified into two types, i.e.,
(1) a method to perform the same processing on all video signals, and
(2) a method to detect specific information on an image and perform processing on only necessary pixels according to the detection result.
The first candidate for the above-mentioned item (1) “method to perform the same processing on all video signals” may be a method to adding dither noise or random noise. Although this method can reduce the false contours to some extent, it has a problem that the added noise component is noticeable.
The second candidate may be a method to use a higher-order bit(s) as the lower-order bit(s) to be added. For example, in order to convert a 6-bit input signal “101101” into an 8-bit input signal, two higher-order bits of the input signal are added as two lower-order bits so that a signal “10110110” is obtained.
The third candidate may be a method to simply add either “0” or “1” as a lower-order bit(s).
Although these second and third methods are simple, they cannot suppress the false contours because they do not bring any improvement in the gray-scale difference in a gray-scale varying portion.
Meanwhile, in a “false contour suppression circuit” disclosed in Patent document 1, a method to perform low-pass filter (LPF) processing on a false contour area (first method) is shown as the above-mentioned item (2) “method to detect specific information on an image and perform processing on only necessary pixels according to the detection result”. In the invention disclosed in Patent document 1, in order to suppress false contours that are produced by performing a gamma correction (image processing) on a digital video signal, an area where false contours occur is adaptively determined, and the integrated value of video signals of pixels located spatially in the vicinity of that area is output (synonymous with LPF processing). This LPF processing reduces the gray-scale difference in the area where false contours occur.
However, there is a problem that if the intervals at which the false contours occur (in other words, contour line intervals) are larger than the filter size (the integrated range of neighboring pixels), a filtered region and an unfiltered region tends to be easily distinguished from each other in a gradation area, and thus not leading to substantial improvement in image quality even though the false contours are suppressed. If the filter size is increased to cope with the problem, it is necessary to prepare a frame memory capable of storing an image data equivalent to an entire screen, and consequently increasing the costs for hardware necessary for the processing.
As a second method, in a “digital signal processing device” disclosed in Patent document 2, a method to reduce false contours that are produced by performing a gamma correction (image processing) is disclosed, in which when a smoothly varying area (gradation area) is determined, gray-scale values of pixels located between contour lines of the false contours in that area are obtained by linearly interpolating gray-scale values of pixels located on the contour lines. This method can realize uniform spatial gray-scale variation within a gradation area, and thus not suffering from the problem of the first method.
From the fact described above, as a method to extend gray-scale, the method in which specific information on pixels are detected and linear interpolation is performed according to the detection result is also desirable in terms of the suppression of false contours. The method using linear interpolation is also disclosed in Patent document 3 “Image processing device, Image processing method, Program and Recording medium”, Patent document 4 “Color signal extending device and Color signal extending method”, and Patent document 5 “Image processing device and Image processing method, and Image display device, Mobile electronic device”.
To begin with, the purpose of performing linear interpolation in the techniques disclosed in Patent documents 3 to 5 is different from that of the techniques disclosed in Patent documents 1 and 2. That is, the gray-scale extension is performed to solve such a problem that false contours occur because the bit depth of a digital video signal is small or to make full use of the gray-scale capability of a display device, rather than suppressing false contours that occur during the image processing. However, the linear interpolation method itself is the same.
The image processing device disclosed in patent document 3 includes a false contour detector and a pixel value convertor. The detection conditions of the false contour detector include a situation where after the same brightness level continues in the horizontal direction over two pixels or more, the brightness level increases by 1 (Condition 1), and a situation where after the brightness level decreases by 1, the same brightness level continues in the horizontal direction over two pixels or more (Condition 2). Then, the pixel value converter performs linear interpolation on a detected false contour.
The color signal extending device disclosed in Patent document 4 includes a data distribution detection unit and a data depth extension unit. The data distribution detection unit extracts (detects), from the data value distribution state, pixels having such a distribution state that the color varies gently. Specifically, it detects an area where, in a pixel group K where the same gray-scale continues, the number of pixels is greater than or equal to a lower-limit threshold P and smaller than or equal to an upper-limit threshold Q, and the gray-scale difference from pixels of an adjacent pixel group is smaller than or equal to a decision threshold S. Then, the data distribution detection unit performs linear interpolation on the area where the color varies gently, and determines a gray-scale value to be added. The data depth extension unit assigns a value corresponding to the value to be added to an extension portion, and generates extended data obtained by extending the data depth of the color signal.
The image processing device disclosed in Patent document 5 includes detecting means and signal extending means. The detecting means determines whether or not the difference between an initial position from which the same pixel data continues and another initial position from which the next pixel data continues is equal to a width over which the same data continues. Further, it also determines whether the image data value of pixels where the same image data continues is greater by 1 or smaller by 1 than the image data value of pixels where the next image data continues. Based on the decision result, the signal extending means extends the gray-scale of the image data smoothly and linearly (as linear interpolation) such that smoothly continuous image is formed in the area where pseudo-contours is occurring.
However, in the inventions disclosed in Patent documents 3 to 5, detection processing is performed only in a specific direction (e.g., horizontal direction) of the image plane. Therefore, in order to obtain a sufficient effect from the gray-scale extension processing, it has been necessary to perform detection processing in two directions. However, to perform detection processing in two directions, it requires a frame memory capable of storing an image data equivalent to an entire screen (i.e., to perform detection processing in a direction perpendicular to the raster direction, it is necessary to temporarily store the image signal in a frame memory), and consequently increasing the costs of the processing device.
In the case where a line memory, which is more inexpensive than the frame memory, is used, the gray-scale extension processing is performed only in one direction (e.g., horizontal direction) on a line-by-line basis.